How to map (large) integer on (small in size( alphanumeric string with PHP? (Cantor?)

Question

Dear all,

I can't figure out how to optimally do the following in PHP: In a database, I have messages with a unique ID, like 19041985. Now, I want to refer to these messages in a short-url service but not using generated hashes but by simply 'calculate' the original ID.

In other words, for example: http://short.url/sYsn7 should let me calculate the message ID the visitor would like to request.

To make it more obvious, I wrote the following in PHP to generate these 'alphanumeric ID versions' and of course, the other way around will let me calculate the original message ID.

The question is: Is this the optimal way of doing this? I hardly think so, but can't think of anything else.

$alphanumString = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_';
for($i=0;$i < strlen($alphanumString);$i++)
{
 $alphanumArray[$i] = substr($alphanumString,$i,1);
}



$id = 19041985;

$out = '';
for($i=0;$i < strlen($id);$i++) {

 if(isset($alphanumString["".substr($id,$i,2).""]) && strlen($alphanumString["".substr($id,$i,2).""]) > 0) {
  $out.=$alphanumString["".substr($id,$i,2).""];
 } else {
  $out.=$alphanumString["".substr($id,$i,1).""];
  $out.=$alphanumString["".substr($id,($i+1),1).""];
 }
 $i++;
}

print $out;

Answer 1

echo trim(base64_encode(pack("L", 19041987)), "=");
print_r(unpack("L", base64_decode("w44iAQ")));

• Pack changes the number into four bytes, which is very short but unreadable.
• Base64_encode changes the four bytes into some more human-readable characters.
• It appends some = characters, which are not needed.

If you do base64_encode(19041987), you get the encoding for the string "19041987", which is not shorter.

Answer 2

You should never use a function inside a for statement since it's played during every loop.

For instance your

for($i=0;$i < strlen($alphanumString);$i++)
{
 $alphanumArray[$i] = substr($alphanumString,$i,1);
}

should be

var $alphaLength = strlen($alphanumString);
for($i=0;$i < $alphaLength;$i++)
{
 $alphanumArray[$i] = substr($alphanumString,$i,1);
}